bug#50571: 28.0.50; Redisplay segfaults with empty face cache

[Eli Zaretskii <eliz@gnu.org>]

> From: "Basil L. Contovounesios" <contovob@tcd.ie>
> Cc: 50571@debbugs.gnu.org
> Date: Sun, 19 Sep 2021 14:57:02 +0100
> 
> Unfortunately I found another hole that needs plugging, but fortunately
> I can reliably reproduce it with the following site-specific steps:
> 
> 0. emacs
> 1. C-x p p (project-switch-project)
> 2. Select a checkout of https://github.com/abo-abo/swiper,
>    using Ivy completion.
> 3. f (project-find-file)
> 4. ivy.el RET
> 5. C-s (isearch-forward)
> 6. C-g
> 7. M-s s (counsel-grep-or-swiper)
> 8. #[[:digit:]]
> 
> This brings a bug-reference-bug-regexp match onto screen, which again
> triggers a frame creation via bug-reference's call to display-warning.
> 
> The attached GDB log shows where the relevant frame's face cache is
> cleared right before the crash (search for 'New value = 0'), at which
> point f->inhibit_clear_image_cache is false.

Thanks for the data.  I guess this is a preview of how allowing
arbitrary Lisp that affects the display in fontification-functions
will keep shooting in the foot, time and again.

Please try the patch below.

diff --git a/src/xdisp.c b/src/xdisp.c
index 2e72f6b..8b56fee 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -4288,12 +4288,17 @@ handle_fontified_prop (struct it *it)
       struct buffer *obuf = current_buffer;
       ptrdiff_t begv = BEGV, zv = ZV;
       bool old_clip_changed = current_buffer->clip_changed;
+      bool saved_inhibit_flag = it->f->inhibit_clear_image_cache;
 
       val = Vfontification_functions;
       specbind (Qfontification_functions, Qnil);
 
       eassert (it->end_charpos == ZV);
 
+      /* Don't allow Lisp that runs from 'fontification-functions'
+	 clear our face and image caches behind our backs.  */
+      it->f->inhibit_clear_image_cache = true;
+
       if (!CONSP (val) || EQ (XCAR (val), Qlambda))
 	safe_call1 (val, pos);
       else
@@ -4327,6 +4332,7 @@ handle_fontified_prop (struct it *it)
 	    }
 	}
 
+      it->f->inhibit_clear_image_cache = saved_inhibit_flag;
       unbind_to (count, Qnil);
 
       /* Fontification functions routinely call `save-restriction'.